--- a/tools/build.py
+++ b/tools/build.py
@@ -284,7 +284,7 @@ def _update_index_package_metadata(index
                 index_package_json["versions"][v].append(metadata.version)
 
 
-def build(output_path, hash_prefix_len, mpy_cross_path):
+def build(output_path, unix_ffi, hash_prefix_len, mpy_cross_path):
     import manifestfile
     import mpy_cross
 
@@ -310,7 +310,7 @@ def build(output_path, hash_prefix_len,
 
     # For now, don't process unix-ffi. In the future this can be extended to
     # allow a way to request unix-ffi packages via mip.
-    lib_dirs = ["micropython", "python-stdlib", "python-ecosys"]
+    lib_dirs = ["unix-ffi"] if unix_ffi else ["micropython", "python-stdlib", "python-ecosys"]
 
     mpy_version, _mpy_sub_version = mpy_cross.mpy_version(mpy_cross=mpy_cross_path)
     mpy_version = str(mpy_version)
@@ -438,6 +438,7 @@ def main():
 
     cmd_parser = argparse.ArgumentParser(description="Compile micropython-lib for serving to mip.")
     cmd_parser.add_argument("--output", required=True, help="output directory")
+    cmd_parser.add_argument("--unix-ffi", action="store_true", help="process unix-ffi packages")
     cmd_parser.add_argument("--hash-prefix", default=8, type=int, help="hash prefix length")
     cmd_parser.add_argument("--mpy-cross", default=None, help="optional path to mpy-cross binary")
     cmd_parser.add_argument("--micropython", default=None, help="path to micropython repo")
@@ -447,7 +448,7 @@ def main():
         sys.path.append(os.path.join(args.micropython, "tools"))  # for manifestfile
         sys.path.append(os.path.join(args.micropython, "mpy-cross"))  # for mpy_cross
 
-    build(args.output, hash_prefix_len=max(4, args.hash_prefix), mpy_cross_path=args.mpy_cross)
+    build(args.output, args.unix_ffi, hash_prefix_len=max(4, args.hash_prefix), mpy_cross_path=args.mpy_cross)
 
 
 if __name__ == "__main__":
